The present invention relates generally to network interfacing, and more particularly, to an apparatus and method for mixing down and spectrum folding a frequency diverse modulated carrier to baseband in a network communications receiver.
The transmission of various types of digital data between computers continues to grow in importance. The predominant method of transmitting such digital data includes coding the digital data into a low frequency baseband data signal and modulating the baseband data signal onto a high frequency carrier signal. The high frequency carrier signal is then transmitted across a network physical transmission medium such as electrical cable, fiber optic, RF, or other medium to a remote computing station.
At the remote computing station, the high frequency carrier signal must be received and demodulated to recover the original baseband signal. In the absence of any distortion across the network medium, the received signal would be identical in phase, amplitude, and frequency to the transmitted carrier and could be demodulated using known mixing techniques to recover the baseband signal. The baseband signal could then be recovered into digital data using known sampling algorithms.
One problem with such networks is that the physical medium and network topology tend to distort the high frequency carrier signal. Branch connections and different lengths of such branches cause reflections of the transmitted signal. Such problems are even more apparent in a network which uses home telephone wiring cables as the network physical medium. The typical wiring of the telephone network is designed for the xe2x80x9cplain old telephone servicexe2x80x9d (POTS) signals in the 3-10 kilohertz frequency and are not designed for transmission of high frequency carrier signals in a frequency range greater than 1 MHz. The high frequency carrier signal is also distorted by transients in wiring characteristics due to on-hook and off-hook switching and noise pulses of the POTS (e.g. ringing). The high frequency carrier is further distorted by spurious noise caused by electrical devices operating in close proximity to the xe2x80x9ccablexe2x80x9d medium.
Such distortion of frequency, amplitude, and phase of the high frequency carrier signal degrades network performance and tends to impede the design of higher data rate networks. Known techniques for compensating for such distortion and improving the data rate of a network include complex modulation schemes and frequency diversity.
Utilizing a complex modulation scheme such as quadrature amplitude modulation (QAM), both the amplitude and phase of the high frequency carrier are modulated to represent I and Q components of a baseband signal. Referring to FIG. 1, a 4-QAM modulation constellation 10 is shown. In operation, each data symbol is represented by an I-value of +1 or xe2x88x921 and a Q-value of +1 or xe2x88x921 such that the data symbol can be represented by one of the four states 12(a)-(d) in constellation 10. Each constellation pointy 12(a)-12(d) represents a unique combination of carrier amplitude and phase. For example, constellation point 12(a) represents a carrier amplitude of 14 and a carrier phase 16.
FIG. 2 illustrates the utilization of frequency diversity by transmitting the same data in three mutually exclusive sub-spectra 18(a)-(c) of the transmission band 20. Therefore, if a portion of the band is distorted (e.g. one or more of the sub-spectra 18(a)-(c)), the data may still be recovered at the receiver from a less distorted portion of the sub-spectra 18(a)-(c). For example, a data signal modulated onto a 7 MHz carrier utilizing 6 MHz of bandwidth may include three mutually exclusive sub-bands 18(a)-(c) centered at 5 MHz, 7 MHz and 9 MHz.
One approach to demodulating such complex signals is to use filters implemented by digital signal processing (DSP), which provides for a convenient way of varying filter coefficients for each transmission to accommodate carrier distortion as detected in the particular time frame in which the data is being transmitted. Using such approach, the receiver compares the distorted received signal representing a known preamble transmission (prior to the data transmission) to the undistorted waveform of the preamble and determines the appropriate filter coefficients for recovery of the received signal. Such filter coefficients are then used for receiving the data transmission.
In accordance with DSP technology, the high frequency carrier is typically sampled with an A/D converter at a rate that is at least 4 times the carrier frequency. Assuming a carrier frequency on the order of 7 MHz, the sampling rate will be on the order of 30 MHz. A problem associated with processing digital samples at such rates to demodulate a complex modulated carrier, and to process mutually exclusive sub-bands of a frequency diverse system, is that very large and costly digital signal processing systems would be required. Therefore, based on recognized industry goals for size and cost reductions, what is needed is a device and method for recovering data signals from a received modulated carrier that do not suffer from the complexity disadvantages of known systems.
A first objective of the present invention is to provide a demodulation circuit comprising: a) an A/D converter running at a sampling frequency and generating a series of samples representing a frequency diverse modulated carrier; b) a mixer operating to mix the series of samples with a sine wave of one fourth the sampling frequency, represented by a series of sine wave values occurring at the sampling frequency, the mixer generates a mixed down signal of a series of samples occurring at the sampling frequency; and c) a decimation filter operating at a decimation factor equal to the sampling frequency divided by the frequency difference between adjacent sub-spectra for folding the sub-spectra and retaining a portion of the mixed down series of samples. The portion of the mixed down series of samples may occur at a slow sampling rate equal to the sampling rate divided by the decimation factor of the decimation filter.
The sine wave may have the same frequency as the modulated carrier such that the mixed down series of samples represents the original frequency diverse baseband signal. Alternatively, the sine wave may have a frequency other than that of the modulated carrier and the circuit may further comprise a second mixer operating to mix the portion of the mixed down series of samples with a second sine wave represented by a second series of sine wave values occurring at the slow sampling rate, the second mixer generating the baseband signal.
The mixed down series of samples may represent the I-channel of a system having complex constellation points. The system may further comprise: d) a Q-channel mixer operating to mix the series of samples with a cosine wave of one fourth the sampling frequency represented by a series of cosine wave values occurring at the sampling frequency, the Q-channel mixer generating a mixed down series of samples occurring at the sampling frequency; and e) a Q-channel decimation filter operating at a decimation factor equal to the sampling frequency divided by the frequency difference between adjacent sub-spectra for folding the sub-spectra and retaining a portion of the mixed down series of samples. Both the sine wave and the cosine wave may have the same frequency as the modulated carrier such that both the sine mixed down series of samples and the cosine mixed down series of samples represent the I-channel and the Q-channel of a frequency diverse baseband signal respectively. Alternatively, both the sine wave and the cosine wave may have a frequency other than that of the modulated carrier and the circuit may further comprise: f) a second mixer operating to mix the portion of the mixed down series of samples with a second sine wave represented by a second series of sine wave values occurring at the slow sampling rate, the second mixer generating a baseband signal; and g) a second Q-channel mixer operating to mix the portion of the cosine mixed down series of samples with a second cosine wave represented by a second series of cosine wave values occurring at the slow sampling rate, the second Q-channel mixer generating a baseband Q-channel signal.
Preferably, the sample frequency is 32 MHz, both the sine wave and the cosine wave have a frequency of 8 MHz, the decimation filter has a decimation factor of 16:1, and both the second sine wave and the second cosine wave have a frequency of 1 MHz.
A second objective of the present invention is to provide a method of folding adjacent sub-spectra of a frequency diverse modulated carrier signal, the method comprising: (a) selecting a sample frequency that is a multiple of the difference between adjacent sub-spectra; (b) sampling the modulated carrier at the sampling frequency to generate a series of samples; and (c) decimating the series of samples utilizing a decimation filter with a decimation factor equal to the sample frequency divided the difference between adjacent sub-spectra to generate a decimated series of samples. The method may further comprise mixing the series of samples with a sine wave with a frequency equal to that of the modulated carrier and represented by a series of values occurring at the sample frequency to down mix the modulated carrier to a baseband signal. The method may further yet include mixing the decimated series of samples with a second sine represented by a series of values occurring at the same frequency as that of the decimated series of samples to mix the decimated series of samples to baseband.